Weak short-latency spinal projections to the long flexor of the human thumb.

The human thumb is controlled by a muscle, flexor pollicis longus (FPL), that is unique among mammals and contributes to manual dexterity. The present study sought to define whether the spinal reflex circuitry for this muscle differed from that for an adjacent muscle (flexor carpi radialis, FCR). In peri-stimulus time histograms, short-latency, largely monosynaptic excitation produced by median nerve stimulation was significantly less frequent and significantly smaller for FPL motor units than FCR motor units. Thus the motoneurone pools of adjacent muscles differ in their spinal reflex accessibility. The reflex control of FPL may thus be achieved by supraspinal pathways rather than the traditional monosynaptic arc.